The spreadsheet can’t be opened “for some reason”?? What sort of error message is that??
“For some reason.” How this ever got past any sort of quality assurance I cannot imagine. Did the engineer/s assign an out-of-bounds error code to the problem, and the operating system can’t decide what to say and so falls back to “for some reason”?
I got the same error message back in April while opening a Pages file from a shared drive after a reboot. No clue what it meant then, and it was fixed with a restart of Pages so it doesn’t seem to overlap with Arthur’s cause of error. It does seem to be specific to iWork apps, though.